Traditionally stock sites allow you download a free, watermarked "comp" image which can be used in your design until the client approves the piece. Once the client has approved, you can then go back and purchase and download the non-watermarked image.
You are free to purchase and download the non-watermarked image whenever you'd like however. It's not unheard of to purchase images to use in design mock-ups before the client approves anything. This is often done so that any watermark does not taint the perception of the final design.
If you use the Media for printed materials, the number of copies must not exceed 500,000.
If more than 500,000 books are being printed, you'd need to look into extended licenses if you happen to use the site where this stipulation exists.